A method of measuring the average luminance of a scene to be photographed and controlling the shutter speed, f-number, and the like of a camera is conventionally known as a method of taking a picture with proper brightness. There is also known an exposure control method using a so-called evaluation photometry method of dividing a scene into predetermined regions, weighting the luminance measured for each region, and calculating the average luminance to obtain the correct exposure.
However, in a so-called back light scene where a main object to be photographed is much darker than the background, the main object becomes dark in a photographed image. To take a picture with proper brightness in this back light scene, the exposure of a camera must be set such that the picture is taken to be brighter than an average picture in photography. This exposure correction operation is troublesome, and a skilled technique is required to set the camera properly. Any user cannot always make settings easily. Even if exposure correction is properly performed for the main object, the background portion may become too bright.
In analog photography, prints with proper brightness can be obtained by performing so-called dodging in a darkroom. To obtain a digital image with proper brightness, it is desirable to implement this dodging in digital image processing.
As a method of implementing this process, a technique for improving an image by, e.g., calculating a difference between a component obtained by logarithmically transforming a digital image and a low-frequency component of the logarithmically transformed component, darkening a bright component in a low-frequency region of the digital image, and brightening a dark component in the low-frequency region (see, e.g., non-patent reference 1).
There is proposed a method of obtaining an effect as that obtained by dodging in digital image processing by using a luminance component and its low-frequency component (see, e.g., non-patent reference 2).
Non-patent reference 1 is Jobson et al., “A Multiscale Retinex for Bridging the Gap Between Color Images and the Human Observation of Scenes”, IEEE TRANSACTIONS ON IMAGE PROCESSING, July 1997, Vol. 6, No. 7, and non-patent reference 2 is Reinhard et al., “Photographic Tone Reproduction for Digital Images”, ACM Transactions on Graphics, July 2002, Vol. 21, No. 3.
However, to extract low-frequency luminance components of a digital image at a plurality of resolutions, particularly as for an image of low resolution, a relatively large region of the original image of the digital image (to be referred to as “original digital image” hereinafter) must be referred to. To refer to the original image, a method of storing the digital image in a storage device can be considered. This method, however, requires a large storage area to store the digital image.
Accordingly, if a large storage area cannot be ensured or a storage area needs to be removed, this method cannot be adopted when, e.g., a scanner device is used.
Under the circumstances, as another method, a method of scanning the original digital image twice, generating a low-frequency component of the digital image by the first scanning, and correcting a digital image obtained by the second scanning using the low-frequency luminance component generated by the first image scanning can be considered.
However, the image obtained by the first scanning may not accurately coincide with the image obtained by the second scanning, depending on the control precision of the head position of a scanner device. In this case, an outline like a blur may occur at an edge portion of the corrected image.